The invention relates to an adaptive predistortion circuit for a digital transmission system for sending input data at the rate of a symbol clock having a period T with the aid of a modulator and a power amplifier which distorts the data, the circuit comprising:
a predistortion circuit for predistorting in the opposite sense the input data prior to their entering the amplifier in order to transmit the expected input data, PA1 an adoption circuit for continuously adapting the predistortion circuit to the stream of input data in response to a demodulation of the stream of transmitted data, PA1 and a filter performing a signal shaping. PA1 on a first path, digital data b.sub.k leaving a first predistortion circuit in a predistorted condition as digital data b.sub.k in-phase with the symbol clock, PA1 on a second path, digital data c.sub.k leaving a second predistortion circuit in a predistorted condition as digital data c.sub.k phase-shift by T/3 with respect to the symbol clock, PA1 on a third path, digital data d.sub.k leaving a third predistortion circuit in a predistorted condition as digital data d'.sub.k phase-shifted by 2T/3 with respect to the symbol clock, PA1 on a first path, by the respective coefficients h.sub.i,0, the products being added together to produce the data b.sub.k, PA1 on a second path by the respective coefficients h.sub.i,1, the products being added together to produce the data c.sub.k, PA1 on a third path by the respective coefficients h.sub.i,2, the products being added together to produce the data d.sub.k. PA1 of the symbols b.sub.k at instants kT PA1 of the symbols c.sub.k at the instants kT+T/3 PA1 of the symbols d.sub.k at the instants kT+2T/3.
The invention finds its applications in the digital transmission systems such as the data transmission modems, radio relay links, space communications systems.
For an efficient use of the available spectrum, the current digital transmission systems, specifically the radio relay links and the systems for transmitting data over the telephone channel, use modulation methods with large numbers of phase and amplitude states. These modulation methods are very sensitive to all types of distortion, and of course to non-linear types of distortion caused by amplifiers, mixers and other non-linear circuits in the transmission chain. A particularly critical point with respect to radio relay links and satellite transmission is the non-linearity of the transmitter power amplifier or the on-board power amplifier in the case of satellite transmissions. These amplifiers are known for their non-linear characteristics. If they are used in their linear zone, the full extent of their power is not utilized. If they are made to operate near to their saturation power level, they will distort the signal in an unacceptable manner. In practice, for a given power amplifier, one fixes the level of the transmitted signal such as to establish a compromise between the signal-to-noise ratio and the non-linear distortion undergone by the signal. Thus the optimum operating point of the amplifier is the one at which the joint effects of the additive noise of the channel and of the non-linear distortion of the amplifier are minimized. For modulation methods with a large number of states (64-QAM and 256-QAM, for example), this point is remote from the saturation power level of the amplifier, which implies that the latter is not used efficiently. In order to enhance its efficiency, predistortion techniques (fixed or adaptive) are currently used which make it possible to reduce the effect of the power amplifier's non-linearity on the transmitted signal.
A currently used predistortion technique consists of inserting in the intermediate-frequency stage of the transmitter a non-linear circuit for realizing an approximation of the inverse function of the power amplifier whose non-linearities one seeks to compensate. If the exact inverse of the function of the amplifier could be synthesized, this technique would make it possible to have a perfect signal at the output (without any non-linear distortion). However, this cannot be realized because the exact inverse would require a circuit of infinite complexity. In practice one is satisfied with making an approximation and in most cases the Taylor series representing the non-linear function of the amplifier is limited to the third order and a predistortion circuit is synthesized, also of the third order, in a manner such that the two cascaded circuits no longer have third-order distortion. Higher-order terms (fifth order and seventh order) appear at the output but have smaller amplitudes compared to the initial third-order distortion. The result is then a certain improvement of the performance of the system. A disadvantage of these predistortion circuits in the intermediate frequency stage resides in the fact that they are analog circuits. They are hard to make adaptive and require from time to time an intervention to readjust them and compensate for the variations of the amplifier response according to time and temperature. This predistortion technique has to be dispensed with if one wishes to have an automatic send power control.
Another more recent predistortion technique consists of modifying the alphabet of data to be transmitted. This technique called "Data Predistortion" or "Baseband Predistortion" is known from U.S. Pat. No. 4,291,277 and from the article by A. A. M. SALEH and J. SALZ "Adaptive Linearization of Power Amplifiers in Digital Radio Systems", Bell System Technical Journal, Vol. 62, April 1983, pp. 1019-1033.
In the article by A. A. M. SALEH and J. SALZ, FIG. 1 is a schematic representation of an adaptive predistortion circuit which supplies to the input of the modulator a distorted constellation on the basis of the original square constellation, for example, an amplitude modulation of two quadrature carriers (QAM). The amplifier acts on the constellation by producing a net compression and a net rotation of the points having large amplitudes. In order to compensate for this effect the original constellation is distorted so that it resumes its original square shape after passing through the power amplifier. Thus, when the distortion circuit is optimized, it forms the inverse of the power amplifier (apart from a constant gain and a constant phase shift) and allows for perfectly compensating for the non-linearities of the amplifier. In order to make this circuit adaptive, the signal is recaptured at the output of the amplifier, demodulated, then sampled at the symbol transmission rate 1/T and these samples are compared to the points corresponding with the QAM constellation used. These comparisons make it possible to obtain a control signal which enables optimization of the predistortion circuit with the aid of a conventional algorithm. However, the scheme used in FIG. 1 is very simplistic because it does not have any filtering before the modulator or before the power amplifier. Thus, it does not correspond with the solution generally used. In effect, in the real systems (cf. U.S. Pat. No. 4,291,277), a spectral signal shaping filtering of the Nyquist type is always used which makes it possible to limit the signal band while a zero intersymbol interference at the decision instants is guaranteed. This filtering is generally equally divided between the send and receive ends so as to maximize the signal-to-noise ratio at the decision instants. In such systems the effect of the non-linearity of the amplifier is twofold: the constellation is not only deformed but intersymbol interference appears, associating a cloud of points to each point of the constellation. With the above-described predistortion technique it is, however, not possible to compensate for this second effect.